The semicontinuous addition of styrene to a low solid-content (6 wt%) polystyrene latex obtained by polymerization of a three-component microemulsion, allows the production of a high solid-content latex (ca. 40 wt%) containing small particles (< 40 nm) of high molecular weight (> 10(6) g/mol). The method can be extended to latex made from microemulsions containing other water-insoluble monomers.